This invention relates to a charge-coupled device (COD) image sensor with a vertical-overflow drain (VOD) for antiblooming protection, and more particularly, to low voltage electronic shuttering of solid-state CCD image sensors.
Prior art structures for improving the electronic shuttering characteristics of image sensors provide confined regions of higher doping concentration directly beneath the individual detector elements of an interline (IL) CCD imager as shown in FIG. 4. This reduces the depletion spreading into the substrate thereby increasing capacitive coupling from the substrate to the so-called overflow point. This increased capacitive coupling allows the overflow barrier (potential difference between the storage region of the photodetector and the overflow point) to be pulled down more easily, i.e., with less voltage on the substrate.
However, processing is made difficult in the prior art by the fact that these regions are confined to the area directly under each individual detector. Implanting these n+ regions relatively late in the process (e.g., around the same time the photodiodes are formed) would require extremely high energy or a reduced p-well depth. A reduced p-well depth would reduce photosensitivity at longer wavelengths, and is therefore undesirable. The currently available high-energy implantation equipment is not capable of providing energies high enough for this approach. Another problem with high-energy implantation is maintaining purity of the beam which makes process control difficult. Implanting these regions early in the process and driving them also would be difficult since registration of various layers would be poor (i.e., self alignment would not be possible).
Prior art CCD image sensors of the type shown in FIG. 3, but not for type shown in FIG. 4, however, typically require relatively large substrate voltages (around 40 volts) to affect electronic shutter action.
The prior art sensor shown in FIG. 9 is illustrated similar as in U.S. Pat. No. 5,014,132.
This prior art device has improved voltage characteristics for electronic shutter operation which results from heavily doping the substrate 91 as compared to n-type layer 92. P-well 93 is provided within which imaging layers are created. However, the device in FIG. 9 does not result in desirable internal gettering characteristics because oxygen precipitation is more difficult to induce when the n-type dopant concentration is increased.
It can be seen from the foregoing discussion that their remains a need within the art for a device having improved low voltage characteristics for the electronic shutter that is built on a substrate that is easy to getter internally.
We have invented a structure that provides a reduction in the voltage required to operate electronic shutters while maintaining sufficiently easy gettering within the sensor. The resulting performance eases the demands on off-chip support circuitry required to operate this sensor. The present invention provides an improved pixel structure that allows for a reduction in this voltage required to operate the electronic shutter. The improved structure is provided with a simplified manufacturing process compared to conventional processes used by other CCD manufacturers. The invention improves upon the prior art, to alleviate the previously discussed difficulties, by providing between an n-type substrate and a p-type well, a uniform, deep n+ layer underneath the entire area of the device as shown in FIG. 2. Although this layer could be implanted early in the process, we have found it more convenient and easier to control by creating the deep n+layer via epitaxial growth. Additionally, the invention provides another n-type layer above the n+ layer.
It is also envisioned, that variations in the doping levels within the layer structures can be employed to achieve low voltage operation while maintaining internal gettering characteristics. This is provided by an image sensor structure that has a substrate of n-type conductivity, a first laterally uniform, deep n-layer on top of the substrate and underneath the entire area of the image sensor and a second laterally uniform n-layer on top the first laterally uniform n-layer that extends to the top surface of the device, and a laterally uniform, lightly doped p-layer formed in the second laterally uniform n-layer, such that the p-layer is located beneath image sensor and above the n-substrate and the first deep n-layer.
The above and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
The present invention has advantages over the prior art providing low voltage shuttering while retaining a simple fabrication process.